The apparatus and method of the invention will be described with reference to the problem of acquiring information concerning the physiological status of the brain by measuring and analyzing variations of pressure within the brain. The invention may also be used to acquire information concerning the physiological status of other types of body organs.
Each time the heart beats, it forces blood out into the arteries of the body. A portion of this blood enters the skull to supply oxygen and nutrients to the brain. The pumping of the blood by the heart creates a pulsation of the blood through the arteries. This pulsation of the blood in the arteries of the brain contributes to the fluid pressure levels in the brain. The fluid pressure in the brain is generally referred to as the "intracranial pressure" or "ICP". The intracranial pressure is dependent upon several factors including the elastic characteristics of the brain, the volume of cerebro-spinal fluid in the brain and the rate of flow of the blood in the brain.
The flow of blood to the brain is automatically regulated by the body to avoid increases in intracranial pressure that might damage the brain. In addition, other regulatory processes in the body operate to keep the level of the intracranial pressure within safe limits. Some of these other regulatory processes include the formation and absorption of cerebro-spinal fluid, maintenance of carbon dioxide levels in the blood, etc. When the brain is subjected to head trauma, internal bleeding, brain tumors or other abnormal conditions, the intracranial pressure may rise to dangerous levels. If the body's regulatory processes are not able to control the increased intracranial pressure, then death may result.
In the treatment of such abnormal brain conditions it is desirable to know the magnitude of the mean value of the intracranial pressure. If the mean intracranial pressure is increasing with time, a worsening of the patient's condition is indicated and remedial steps may be taken. Techniques currently in use for monitoring changes in intracranial pressure involve the placement of a pressure monitoring device into the skull of the patent to directly measure the actual pressure inside the patient's head. As the blood pulses through the brain, an intracranial pressure waveform is produced. The mean value of the intracranial pressure waveform is taken to be the mean value of the intracranial pressure.
The mean value of the intracranial pressure, however, defines only one parameter in a complex systems of forces operating within the brain. As mentioned above, increases in the intracranial pressure may also be compensated for by changes in carbon dioxide levels in the blood, rate of blood flow, cerebro-spinal fluid formation and absorption and other factors. Because of the interrelationship between these factors, the mean value of the intracranial pressure often shows changes only after the compensating mechanisms have failed. For this reason there have been many attempts made to develop techniques for measuring and monitoring both the cerebral blood flow and the elastic properties of the brain.
The elastic capacity of the brain is referred to as the "cerebral compliance". Compliance is defined as the change in pressure per unit volume of added fluid. One technique currently in use for the measurement of the compliance of the brain involves the injection of fluid into the patient's intracranial cavity and the direct measurement of the resulting changes in the intracranial pressure. This technique can provide valuable information concerning the degree of severity of a brain injury. However, the technique has severe limitations due to the fact that injections of fluid into the brain may produce dangerous increases in intracranial pressure. Risks of infection are also present. In addition, the injection of fluid may only be done a very few times (often only once every day or two) with the risk of infection and other complications increasing with every injection. For these reasons this technique has not been widely practiced. There currently exists no known prior art system for acquiring information concerning cerebral compliance on a continuous on-line real time basis.
In addition, no known prior art technique exists for measuring the changes in the regulation of cerebral blood flow on a continuous on-line real time basis. Complex techniques exist for performing a one time measurement of cerebral blood flow using either injectable tracers or absorbable gases. These techniques, however, are not useful for the continuous monitoring of cerebral blood flow changes.